Coating of automobile bodies is generally conducted by applying electrodeposition paint as the undercoat, thereafter applying intermediate coat and further applying top coat thereon.
As application means of the top coat, for example 1-coat-1-bake method comprising applying a top coating paint and heat-curing the same, or 2-coat-1-bake method using two kinds of top coat paint, i.e., a base coating composition and a clear coating composition, which comprises first applying the base coating composition, applying thereon the clear coating composition without curing the base coating film, and simultaneously heat-curing the two coating films, can be named.
Of these methods, in such 2-coat-1-bake system, it has been heretofore widely practiced to improve appearance of the coating film by using as the base coating composition one containing effect pigment such as aluminum flake, mica and the like.
Effect pigment normally has laminar or flaky structure and is oriented in the coating film in parallel with the substrate surface to glitter brightly and change color tone according to viewing directions, whereby forming a coating film of unique color appearance. Of such properties, the one of changing the color tone according to viewing directions is referred to as flip-flop property (FF property), greater changes in color tone signifying higher flip-flop property and better brilliance of the coating film containing the effect pigment.
On the other hand, switching to water-borne paint is promoted in these years for prevention of atmospheric pollution and conservation of resources, and development of effect pigment-containing, water-borne base coating compositions capable of forming coating film exhibiting excellent brilliance is being vigorously attempted.
As a means for improving brilliance of base coating film formed of an effect pigment-containing, water-borne base coating composition, methods for increasing pigment weight concentration (hereafter may be abbreviated as “PWC”) of effect pigment in the solid paint content have been investigated. However, when PWC of effect pigment in a water-borne base coating composition is increased, the coating film formed therefrom tends to have deteriorated gloss or surface smoothness, inviting such problems as failure to provide satisfactory finished appearance or reduction in coating film performance such as water resistance.
As a means to deal with the problems, for example, JP 2002-179988A discloses a water-borne base coating composition comprising amido-containing acrylic resin and 15-50 wt % as PWC of aluminum pigment containing 0.1-50 wt % as PWC of aluminum pigment having an average particle size of 17-60 μm. However, brilliance and surface smoothness of the coating film are still insufficient when this water-borne base coating composition is used, and also the coating film performance such as water resistance is unsatisfactory.
JP2004-351390A discloses a coating method comprising recoating on a coating film of water-borne base coating paint containing effect pigment at a relatively low PWC, another water-borne base coating paint containing effect pigment at a relatively high PWC, and further recoating thereon a clear paint, whereby forming a coating film having excellent appearance and brilliance.
Furthermore, JP2004-351389A discloses a coating method comprising recoating on a coating film of water-borne base coating paint containing effect pigment at a relatively low PWC and having a relatively high solid paint content, another water-borne base coating paint containing effect pigment at a relatively high PWC and having a relatively low solid paint content, and further recoating thereon a clear paint, whereby forming a coating film of excellent appearance and brilliance.
However, in either of these coating methods the brilliance and surface smoothness of the resulting coating film are insufficient and the coating film performance such as water resistance is unsatisfactory.